Conventional vehicles use an internal combustion engine to generate input torque, which is ultimately delivered to various gear sets and clutches of a transmission. In order to reduce idle fuel consumption, some engine-driven vehicles are configured with a powertrain that provides engine autostop/start functionality. In such a design, a controller is used to automatically shut off the engine during extended idling periods, for instance when the vehicle waits at a traffic light or in heavy traffic.
To restart the engine after an engine autostop event, an auxiliary starter motor may be used to crank the engine and initiate resumption of the fuel combustion process. However, hydraulic energy is needed during engine restart and subsequent vehicle launch. Therefore, an auxiliary fluid pump may be used to pre-charge a fluid circuit and thereby provide standby fluid pressure at a level needed for actuating the various hydraulic clutches of the transmission upon engine restart.